Special prices if a certain product in cart

E.g. Product A - Normal price: 30 Special price: $15
Product B - Normal price: $7

Scenario 1: In cart, only Product A -> order total: $30
Scenario 2: In cart, Product A + Product B -> order total: $15 + $7 = $22
Scenario 3: In cart, Product A x 2 + Product B -> order total: $15x2 + $7 = $37

Product B is like a 1-time membership fee. When it is added, products will use discounted fixed prices when checkout.

I know the default opencart feature does not support this. But is there any extension that can achieve this?

Thanks.

Absolutely. If you haven't found a module that will work for you, I'd be happy to quote building one for you. If interested, feel free to PM!